win32: get roots on demand for file dialog

collects/mred/private/path-dialog.rkt

@@ -689,15 +689,18 @@
                     (define editor (send this get-editor))
                     (define/public (lock)   (send editor lock #t))
                     (define/public (unlock) (send editor lock #f))
+		    (define/override (on-popup e)
                       (when win?
                         (let ([m (send this get-menu)])
+			  (for-each (lambda (i) (send i delete))
+				    (send m get-items))
 			  (for-each (lambda (r)
 				      (define l (path->string r))
 				      (make-object menu-item% l m
 						   (lambda _
 						     (enter-text r)
 						     (do-enter))))
-                                  (filesystem-root-list))))
+				    (filesystem-root-list)))))
                     (lock))])
           (if win?
             (new c [label #f] [parent this] [init-value ""] [choices '()])